Only your child files I-485 for adjustment of status or DS 260 (for immigraiton visa application) when the PD is current, then his/her CSPA age can be frozen.
You can calculate your child CSPA age when the I-130 (F2A) PD is current: CSPA age = the child actual age when the PD is current - I-130 pending time. If CSPA age is < 21, your child is not age-out and the I-130 is till F2A, thus your child can file I-485 or DS 260 to get the green card. If > 21, your I-130 will be F2B (need to ask USCIS to upgrade F2A to F2B).
Note: I-130 pending time =I-130 aprpoval date (i.e. I-1`30 approval notice issued date) - I-130 receipt date.
